Biomedical Engineering Reference
In-Depth Information
Nanocrystalline materials exhibit quite different properties from
both crystalline and amorphous materials, due to structure, in which
extremely ine grains are separated by what some investigators
have characterized as “glass-like” disordered grain boundaries. The
mechanism of amorphous phase formation by MA is due to a chemical
solid state reaction, which is believed to be caused by the formation
of a multilayer structure during milling [45]. When a mixture of
elemental powders is milled, the formation of the amorphous phase
is due mainly to an ultimate interdiffusion of atoms that occurs at
fresh surfaces and interfaces created by mechanical milling. This
interdiffusion is promoted by defects and chemical disorder in the
crystalline structure.
The basic process of mechanical alloying is illustrated in
Fig. 4.5. Microcrystalline powders are placed together with a number
of hardened steel or tungsten carbide (WC) coated balls in a sealed
container which is shaken. The most effective ratio for the ball to
powder masses is ten.
Figure 4.5
Schematic cross-sectional representation of MA process for
synthesizing nanometer-sized powders (SPEX 8000 mixer
mill).
Furthermore, for all nanocrystalline materials prepared by
a variety of different synthesis routes, surface, and interface,
contamination is a major concern. In particular, during mechanical
attrition, contamination by the milling tools (iron) and atmosphere
(trace elements of O
2
and other rare gases) can be a problem (see
Fig. 4.4) [24, 43]. By minimizing the milling time and using the
purest, most ductile metal powders available, a thin coating of the
milling tools by the respective powder material can be obtained
which reduces Fe contamination tremendously. Atmospheric
contamination can be minimized or eliminated by sealing the vial
with a lexible “O” ring after the powder has been loaded in an inert
gas glove box.
